Global Navigation Satellite Systems (GNSS) such as the U.S. NAVSTAR Global Positioning System (GPS), the European Galileo system, the Chinese Beidou system, and the Russian GLONASS system are increasingly relied upon to provide synchronized timing that is both accurate and reliable. (Reference is made to GPS below, by way of example and simplicity, but similar characteristics and principles of operation apply to other GNSS.) GPS antennas are used to receive GPS signals and provide those signals to a GPS receiver. GPS antennas may amplify and filter the received GPS signals prior to passing them to the GPS receiver. The GPS receiver can then calculate position, velocity, and/or time from the signals collected by the GPS antenna.
Accurate GPS-based navigation and timing systems typically rely on receiving signals from at least four GPS satellites simultaneously. GPS timing systems can provide time when a single GPS satellite is observed if the position of the antenna is already known. Analysis has shown that a GPS timing antenna with a half power beam width (HPBW) of 60° will have access to at least three satellites 95% of the time, which is sufficient for timing applications. GPS satellites transmit right-hand circularly polarized (RHCP) signals, and thus, GPS antennas must be right-hand circularly polarized.
GPS timing antennas at fixed sites are susceptible to unintentional interference, such as out-of-band and multipath signals, as well as intentional interference from ground-based GPS jammers commonly employed to deny, degrade, and/or deceive GPS derived position and time to prevent GPS tracking of commercial or privately owned vehicles.
Several types of antennas have been previously developed to mitigate interference while maintaining a sufficient RHCP HPBW for GPS applications, such as large antenna arrays, horizon ring nulling antennas, and shorted annular ring antennas. Many of these steer a null (local gain minimum) in the direction from which interfering signals are received (such as the horizon). For example, large antenna arrays, such as controlled reception pattern antennas (CRPA), steer a null in the direction of the interference using active circuitry. While CRPAs can achieve exceptional nulling in a particular direction, they can be large due to the multiple antenna elements that are necessary for null steering, are typically expensive due to the required active electronics, and can only null a finite number of interfering signals.
Horizon ring nulling (HRN) antennas can achieve a measured RHCP null depth (i.e., zenith-to-horizon gain ratio) of approximately −45 dB on average around the entire azimuth. The HRN is composed of a shorted annular ring patch combined with a circular patch with amplitude and phase weighting to create a null at the horizon. While the HRN's performance is exceptional with regard to its horizon nulling capability, its cost is relatively high due to the required active electronics. Additionally, the exceptional null of the HRN only applies to incident RHCP interference and not to other polarizations like vertical linear, horizontal linear, or left-hand circular polarization (LHCP).
The quadrifilar helix antenna has been researched extensively for GPS and other applications. Typical short helix antennas have a zenith-to-horizon ratio that is insufficient for horizon nulling, and long helix antennas that may have sufficient nulling at the horizon do not have sufficient HPBW for timing reception.